Import memory from the other module with wasi

Hello! I'm trying to share wasm memory between 2 instances. Could you help me? I've prepared an example that shows (for me) some strange behaviour.

Let's assume that we want to create 2 wasm files -- `library.wasm` (a bunch of useful functions/types/etc that we want to build once) and `user_code.wasm` (a few functions that will interact with the contents of the library that we want to rebuild multiple times). Functions from `library.wasm` and `user_code.wasm` should be able to pass pointers to some data or objects to each other and call their methods from each other, so we have to provide the same memory space for them.

Here is an example of `library.cpp` file:
```
#include
#include

extern "C" {

EMSCRIPTEN_KEEPALIVE
int new_int_ptr() {
std::cout << "############# library new_int_ptr() called #############" << std::endl;
int *x = new int(42);
return (int) x;
}

int main() {
std::cout << "############# library main() called #############" << std::endl;
return 123;
}
}
```

An example of `user_code.cpp` file:
```
#include
#include

extern "C" {

EM_IMPORT(new_int_ptr)
int new_int_ptr();

EMSCRIPTEN_KEEPALIVE
int get_new_int() {
int x = new_int_ptr();
return *(int *) x;
}

int main() {
std::cout << "############# user code main() called #############" << std::endl;
return 101;
}
}
```

And `main.go` file:
```
package main

import (
"fmt"
"io"
"os"
"testing"

"github.com/bytecodealliance/wasmtime-go/v14"
)

var t = &testing.T{}

func ReadWasmBytes(engine *wasmtime.Engine, path string) []byte {
wasmFile, _ := os.Open(path)
wasm, _ := io.ReadAll(wasmFile)
wasmtime.ModuleValidate(engine, wasm)
return wasm
}

func main() {
engine := wasmtime.NewEngine()
store := wasmtime.NewStore(engine)

libraryWasmBytes := ReadWasmBytes(engine, "library.wasm")
userCodeWasmBytes := ReadWasmBytes(engine, "user_code.wasm")

libraryModule, _ := wasmtime.NewModule(engine, libraryWasmBytes)
userCodeModule, _ := wasmtime.NewModule(engine, userCodeWasmBytes)

wasiConfig := wasmtime.NewWasiConfig()
wasiConfig.InheritArgv()
wasiConfig.InheritEnv()
wasiConfig.InheritStdin()
wasiConfig.InheritStderr()
wasiConfig.InheritStdout()
store.SetWasi(wasiConfig)

linker := wasmtime.NewLinker(engine)
linker.DefineWasi()

libraryInstance, _ := linker.Instantiate(store, libraryModule)

libraryMain := libraryInstance.GetFunc(store, "_start")
_, err := libraryMain.Call(store)
fmt.Println("Returned error from library main():", err)

linker.DefineInstance(store, "env", libraryInstance)

linker.AllowShadowing(true)

proc_exit := wasmtime.WrapFunc(store, func(a int32) {
fmt.Println("@@@@@@@@@@@@@ proc_exit() called @@@@@@@@@@@@@")
})
linker.Define(store, "wasi_snapshot_preview1", "proc_exit", proc_exit)

fd_write := wasmtime.WrapFunc(store, func(fd, iovsPtr, iovsLen, nwrittenPtr int32) int32 {
fmt.Println("@@@@@@@@@@@@@ fd_write() called @@@@@@@@@@@@@")
return 0
})
linker.Define(store, "wasi_snapshot_preview1", "fd_write", fd_write)

userCodeInstance, _ := linker.Instantiate(store, userCodeModule)

userCodeMain := userCodeInstance.GetFunc(store, "_start")
_, err = userCodeMain.Call(store)
fmt.Println("Returned error from user code main():", err)

get_new_int := userCodeInstance.GetFunc(store, "get_new_int")
ans, err := get_new_int.Call(store)
fmt.Println("Returned answer and error from get_new_int():", ans, err)
}
```

And the commands that I use to run the code:
```
emcc library.cpp -o library.wasm -s "EXPORTED_RUNTIME_METHODS=['ccall']" -O2
emcc user_code.cpp -Wl,--import-memory -o user_code.wasm -s "EXPORTED_RUNTIME_METHODS=['ccall']" -O2
go run main.go
```

If I run these commands I'll get this output:
```
############# library main() called #############
Returned error from library main(): error while executing at wasm backtrace:
0: 0x10bf - !
1: 0xf36 - !

Caused by:
Exited with i32 exit status 123
Returned error from user code main(): error while executing at wasm backtrace:
0: 0x1ad70 - !
1: 0x4218 - !
2: 0x860 - !
3: 0xb47 - !

Caused by:
0: memory fault at wasm address 0xffffffff in linear memory of size 0x1000000
1: wasm trap: out of bounds memory access
Returned answer and error from get_new_int(): error while executing at wasm backtrace:
0: 0x10bf - !
1: 0x10b2 - !
2: 0x4f17 - !
3: 0xfc4c - !
4: 0xb5d - !
5: 0xb3c - !

Caused by:
Exited with i32 exit status 1
```

Let's look at it more thoroughly. The first line tells us that `main()` from library was successfully called. The next returned error `Exited with i32 exit status 123` is okay too (as far as I understand). But the next error is not: `memory fault at wasm address 0xffffffff in linear memory of size 0x1000000` -- it occurs when we try to call `main()` from the user code. We are not also getting here the log `fd_write() called` for some reason (despite we have changed `wasi_snapshot_preview1.fd_write`). And the next error tells us that `get_new_int()` was called unsuccessfully too (and also our implementation of `wasi_snapshot_preview1.proc_exit` was not called).

But if we comment out `std::cout << "############# user code main() called #############" << std::endl;` line from the user code, the output will change a lot:
```
############# library main() called #############
Returned error from library main(): error while executing at wasm backtrace:
0: 0x10bf - !
1: 0xf36 - !

Caused by:
Exited with i32 exit status 123
@@@@@@@@@@@@@ proc_exit() called @@@@@@@@@@@@@
Returned error from user code main(): error while executing at wasm backtrace:
0: 0xfa - !

Caused by:
wasm trap: wasm `unreachable` instruction executed
############# library new_int_ptr() called #############
Returned answer and error from get_new_int(): 42
```

Now all the functions have worked as expected: `main()` from user code was finished with `proc_exit() called` message (the `` `unreachable` instruction executed`` message is expected as far as I understand), the function `new_int_ptr()` was called successfully and we have observed that the instances use the same memory -- the number `42` was created on the library side and then read on the user code side by its pointer.

By changing this example a bit it is possible to face other errors, so I guess there is some kind of undefined behaviour in this example, but I don't know what causes the problem. Can you help me, please?